MEASURING WATER USE IN A GREEN ECONOMY - UNEP

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turbines, net storage in reservoirs) to Net Ecosystem Accessible Water. 4.2.7 Water and river ecosystems Rivers are more than a freshwater resource and wastewater sink. The rivers network connects various ecosystems within the river basin and this interaction in turn influences the ecological potential of the rivers themselves. In addition to water stocks and flow accounts presented in ECWA, so-called ‘Green Infrastructure Accounts’ propose the calculation of Rivers Ecosystem Potential (REP) as a supplement to the Landscape Ecosystem Potential (LEP) based on terrestrial features. The measurement unit for REP is not m 3 of water but the standard-river-kilometre (srkm) which equals 1 km*m 3 *second -1 . This unit allows comparison and adding up of rivers of different sizes in a meaningful way. It is proposed in SEEA-<strong>WATER</strong> 2007 for water quality accounts. In ECWA, water quantity will therefore appear three times: in the water stock account (in m 3 , mostly aquifers) to calculate the accessible freshwater in stocks (e.g. net of non-usable saline water), in flow accounts (in m 3 ) to estimate the accessible water flow net of the water requested for diluting waste water pollution, and finally as a component of REP (in srkm). The Rivers Ecosystem Potential records (in srkm) amounts of large, medium and small rivers as well as brooks and streams. The stocks (in srkm) are weighted in a second step with a river integrity composite index which combines water quality, fragmentation and an index of the naturalness of river ecotones (the zones between two major ecological communities). The River Ecosystem Potential is correlated with water accessibility over space, in particular regarding terrestrial nature (srkm/ km²) as well as with most of the regulating and socio-cultural services described in table 4.1 (outlined in red). 4.3 Water scarcity indicators Many indices exist to evaluate water resource availability quantitatively. This report considers only models and approaches on a wider panregional scale. These provide initial guidance which may be followed up by more in-depth analysis of the local or regional situation. The major types of indicator considered relate water availability to human requirements or the amounts withdrawn to the renewable water supply. A third group aims to assess the status of water ecosystems and their management against a variety of criteria. 4.3.1 Indices focusing on human water requirements The Falkenmark water stress indicator (Falkenmark, 1989) relates water availability to human population. Falkenmark proposed 1 700 m 3 of renewable water resource per capita per year as the threshold, based on estimates of water requirements in the household, agricultural and energy sectors, and the needs of the environment. Falkenmark defined three levels of scarcity: availability of less than 500 m 3 per capita is designated as ‘absolute scarcity’, 500–1 000 m 3 per capita as ‘water scarcity’ and 1 000–1 700 m 3 per capita as ‘water stress’, with ‘no stress’ above 1 700 m 3 . The Falkenmark indicator is widely used since the data are readily available and the meaning is intuitive and easy to understand. However, it has important disadvantages. Annual national averages hide important scarcity information at smaller scales and the cut-off points 1 000 and 1 700 m 3 per capita are not well founded on physical/ecological evidence. In addition, the indicator does not take into account the availability of infrastructure that modifies the availability of water to users and it has to be specified whether the figures included or exclude green water. In general, indicators based on availability per capita fail to identify problems in areas such as the Murray-Darling basin in Australia, where concerns were based on over-exploitation of 54
Measuring water use in a green economy the resource not linked to population. Other indicators of human water requirements include Gleick (1996) who proposed 50 litres of water per day as the ‘basic human water requirement’ (for drinking, sanitation, bathing and food preparation) and estimated the percentage of individuals in each country who fell short of this threshold. Ohlsson (2000) built on the Falkenmark indicator by integrating a society’s ‘adaptive capacity’: its ability to alter overall freshwater availability using economic, technological or other means. The United Nations Development Programme’s Human Development Index is used to weight the Falkenmark index and Ohlsson (2000) called this a ‘Social Water Stress Index’. 4.3.2 Water resource vulnerability indices Shiklomanov (1991) led the first attempts to consider the ratio of demand to availability. He compared national water demand in the industrial, agricultural and domestic sectors to national annual water availability. Raskin et al. (1997) built on this work, replacing water demand with water withdrawal, and defining the ‘Water Resources Vulnerability Index’ as total annual withdrawals as a percentage of available water resources. Water withdrawals are defined as the amount of water taken out of rivers, streams or aquifers to satisfy human needs for water. The authors suggest that a country is scarce if annual withdrawals are between 20 per cent and 40 per cent of annual supply, and severely waterscarce if the figure exceeds 40 per cent. Alcamo et al. (2000) also use this definition for their ‘criticality ratios’: the ratio of water withdrawals for human use to total renewable water resources. Alcamo et al. apply the ratios using their global model WaterGap. Vorosmarty et al. (2005) used a similar approach, introducing indices for local relative water use and water reuse and applying them using geospatial tools at 8 km resolution. Water use was represented by local demand: the sum of domestic, industrial and agricultural water withdrawals. Dividing local demand by the river corridor discharge entering from upstream cells yields an index of local relative water use. A high degree of stress is indicated when this ratio exceeds 0.4. Total water use from all cells divided by the river corridor discharge gives the water reuse index, which represents the extent to which runoff is recycled or reused as it accumulates and flows toward the basin mouth. Pfister et al. (2009) proposed a water scarcity index, which is often used as a characterisation factor for water consumption in life-cycle impact assessment (LCIA). Using the withdrawal-to-availability ratio calculated for each river basin, a weighting factor is applied for each river basin to account for variation in monthly or annual flows. Finally, a logistic transformation of the transformed withdrawalto-availability values gives a water scarcity index. The water exploitation index (WEI), based on data collected via a joint OECD–Eurostat questionnaire, also uses the ratio of annual total water abstraction to available long-term freshwater resources. A WEI above 20 per cent implies that a water resource is under stress, and more than 40 per cent indicates severe stress and clearly unsustainable use of the resource (Raskin et al., 1997). Most withdrawal-to-availability indicators are based on average volumes per year, which do not reveal sensitive seasonal effects and periods when availability and demand differ. For example, many of the systems cited above would not indicate as stressed or sensitive those areas with high winter rainfall but low water ‘storage capacity’ (e.g. calcareous soils in the Mediterranean). Pfister et al. (2009) include monthly variability, and for European applications the European Environment Agency is currently developing an application of the WEI that disaggregates the information at the monthly and river-basin levels (EEA, 2012). Concern can be expressed that withdrawal-toavailability indicators: 55
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MEASURING WATER USE IN A GREEN ECONOMY - UNEP

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